Application of a High-Efficiency Cabin Air Filter for Simultaneous

Jan 28, 2014 - equipped with cabin air filters but their filtration efficiency ... The HECA filters offered an average in-cabin UFP reduction of 93%, ...
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Application of a High-Efficiency Cabin Air Filter for Simultaneous Mitigation of Ultrafine Particle and Carbon Dioxide Exposures Inside Passenger Vehicles Eon S. Lee and Yifang Zhu* Department of Environmental Health Sciences, Jonathan and Karin Fielding School of Public Health, University of California, Los Angeles, California 90095-1772, United States S Supporting Information *

ABSTRACT: Modern passenger vehicles are commonly equipped with cabin air filters but their filtration efficiency for ultrafine particle (UFP) is rather low. Although setting the vehicle ventilation system to recirculation (RC) mode can reduce in-cabin UFPs by ∼90%, passenger-exhaled carbon dioxide (CO2) can quickly accumulate inside the cabin. Using outdoor air (OA) mode instead can provide sufficient air exchange to prevent CO2 buildup, but in-cabin UFP concentrations would increase. To overcome this dilemma, we developed a simultaneous mitigation method for UFP and CO2 using high-efficiency cabin air (HECA) filtration in OA mode. Concentrations of UFP and other air pollutants were simultaneously monitored in and out of 12 different vehicles under 3 driving conditions: stationary, on local roadways, and on freeways. Under each experimental condition, data were collected with no filter, in-use original equipment manufacturer (OEM) filter, and two types of HECA filters. The HECA filters offered an average in-cabin UFP reduction of 93%, much higher than the OEM filters (∼50% on average). Throughout the measurements, the in-cabin CO2 concentration remained in the range of 620− 930 ppm, significantly lower than the typical level of 2500−4000 ppm observed in the RC mode.

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INTRODUCTION Several epidemiological and toxicological studies have demonstrated the adverse health effects of ultrafine particles (UFPs, diameter ≤100 nm).1,2 The cardiovascular system is considered a target to which UFPs can translocate from the lungs, as demonstrated in animal models3−7 and human subjects.8,9 Once inhaled, UFPs can penetrate the epithelium, enter the circulatory system, and be deposited into secondary organs such as heart, liver, kidney, and brains.1,2,10 The small size and large surface area of UFPs also allow them to penetrate cell walls and localize in mitochondria.11 As a result, UFP exposures can amplify the risk of genetic mutations.12 Because of the numerous redox-active chemicals present in motor vehicleemitted UFPs, systemic inflammation can also occur.13,14 A recent study reported that UFPs have a significant correlation to acute health responses.15 In an urban environment, vehicle emissions usually constitute the most significant source of primary UFPs.16,17 The on-road UFP concentration typically ranges from 10 000 to 500 000 particles/cm3,12,18,19 1 or 2 orders of magnitude higher than a typical ambient level in an urban environment. Despite the short average commuting time (1.3 h/day),20 incabin exposure alone accounts for up to 45−50% of the total daily exposure to UFPs.18,21 Modern vehicles are commonly equipped with cabin air filters;22 however, the overall passenger protection against © 2014 American Chemical Society

UFPs is limited to 40−60% under outdoor air (OA) mode and the filtration efficiency varies as a function of particle size.22,23 The level of protection also varies with respect to the vehicle type and age in addition to the ventilation settings.18 Although operating the automotive ventilation system under recirculation (RC) mode can achieve a protection of ∼90% using the manufacturer-installed cabin air filters,18,24 it also causes passenger-exhaled CO2 to accumulate rapidly in the vehicle cabin.25,26 Exposures to high CO2 concentration of 2500 ppm can significantly reduce decision-making performances.27 Therefore, it is important to reduce both UFPs and CO2 concentrations simultaneously inside vehicles. Installation of an auxiliary in-cabin air filtration system was recently suggested but it requires a considerable level of modification on vehicles.26 This study aimed to achieve a simultaneous reduction of both UFPs and CO2 by applying high-efficiency cabin air (HECA) filtration in the existing OAmode automotive ventilation system. Field measurements were conducted in 12 different vehicles of different models and types from several automobile manufacturers. In-cabin UFP reductions were compared under three driving conditions (i.e., Received: Revised: Accepted: Published: 2328

November 6, 2013 January 14, 2014 January 28, 2014 January 28, 2014 dx.doi.org/10.1021/es404952q | Environ. Sci. Technol. 2014, 48, 2328−2335

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Table 1. Test Vehicle Model Specifications vehicle type

maker

model

year

mileage (km)

filter housing locations

cabin volumea (m3)

hatchback

Ford Toyota Chevrolet Honda Hyundai Nissan Toyota Volkswagen Ford Toyota Honda Toyota

Focus Prius Impala Accord Sonata Sentra Camry Jetta Explorer Highlander Odyssey Sienna

2012 2012 2012 2011 2013 2012 2012 2012 2013 2012 2010 2011

51,347 9,102 1,339 51,194 21,712 30,398 1,931 14,917 16,510 10,611 38,622 74,174

glovebox glovebox gloveboxb glovebox glovebox under dash glovebox under hood glovebox glovebox glovebox glovebox

2.92 3.28 3.51 3.43 3.28 3.11 3.34 3.11 4.89 4.43 7.03 5.76

sedan

SUV minivan

Cabin volume is given as the sum of interior and cargo volumes. bChevrolet Impala 2012 model was not equipped with a cabin air filter but the filter housing was located behind the glovebox.

a

from 2.92 to 7.03 m3.19 To minimize the potential variability that can result from vehicle aging, no vehicles older than 3 years were selected for testing. The accumulated mileage of the vehicles ranged from 1339 to 74 174 km. The cabin air filter housing was most commonly found behind the glovebox, but a few vehicle models also had cabin air filters under the dashboard or hood, as noted in Table 1. Except for the 2012 Chevrolet Impala, all test models were equipped with an in-use OEM filter. Field Measurements. During the field measurements, the in-cabin and on-road concentrations were concurrently monitored for UFPs, PM2.5, black carbon (BC), and CO2 to assess the in-cabin passenger exposure reductions. Two sets of instruments were deployed for the concurrent measurements of the in-cabin and on-road air. While both sets of instruments were located inside the passenger cabin, one set monitored at the center of the passenger cabin and the other set sampled the ambient air. The ambient and on-roadway aerosols were sampled through 3 mm (i.d.) isokinetic probes mounted on the car window. The window gaps were sealed with heavy duty duct tape similarly to how we did previously.18 A similar probe was used for in-cabin air sampling to compensate for diffusion loss in the sampling lines. Two condensation particle counters (CPCs) were deployed to measure the UFP concentrations for the in-cabin (Model 3785, TSI Inc., St. Paul, MN) and on-road (Model 3786, TSI Inc.) conditions. Similarly, two DustTrak (Model 8520, TSI Inc.) and two Qtrak monitors (Model 8554, TSI Inc.) simultaneously measured the in-cabin and on-road concentrations of PM2.5 and CO2. The BC concentrations inside and outside the cabin were also recorded with two aethalometers (Models AE-22 and AE-42, Magee Scientific Co., Berkeley, CA). Along with pollutant concentration measurements, the ventilation airflow rate was continuously monitored with a ventilation meter (Q-trak model 7565-X with model 960, TSI Inc., Shoreview, MN). The hot-wire anemometer probe of the ventilation meter was secured on a single air inlet diffuser while all other diffusers were closed and sealed. All the instruments were calibrated prior to their deployment for field sampling and set to a logging interval of 1 s, except for the aethalometers, which were set to their minimum logging interval of 1 min. In addition, the particle size distribution data were collected using two sets of scanning mobility particle sizers (SMPSs, Model 3080 with Model 3085, TSI Inc., St. Paul, MN) inside the SUVs and minivans. The in-cabin and on-road particle size

stationary, on local roadway, and on freeway) with four different filtration scenarios: no filter, in-use original equipment manufacturer (OEM), and two prototypes of HECA filters.

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METHODS

High-Efficiency Cabin Air (HECA) Filters. Since highefficiency filters are not currently marketed for passenger vehicles, two types of automotive HECA filters (noted here as HECA A and HECA B) were developed in collaboration with an industrial partner. The two HECA filters were similar to OEM cabin air filters in terms of their structure, i.e., the pleated panel type, but differed in the filtration media. Whereas OEM filters are typically composed of a single fibrous layer, the developed HECA filters were manufactured with a double layer, with synthetic fibers on the upstream side and glass fibers on the downstream side. The application of synthetic fibers with different physicochemical properties (e.g., diameter, material, and density) on the upstream layer allows the HECA filters to achieve significantly higher filtration efficiency than OEM filters. The HECA A filters were designed to maintain a pressure drop equivalent to the OEM filters by increasing the intrinsic surface area with 1−3 μm diameter fibers. The HECA B filters were designed to maximize the filtration efficiency using 0.4−0.8 μm diameter fibers. Figures S1 and S2 in the Supporting Information present the SEM images of the two HECA filters, respectively. Because there is no filtration efficiency rating standard currently available for automotive cabin filters, the HECA filters were graded using the minimum efficiency reporting value (MERV) standard28 developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) for building heating, ventilating, and air-conditioning (HVAC) systems. When challenged with 0.3 μm potassium chloride (KCl) particles, the HECA A and B filters achieved an averaged filtration efficiency of 92% and 99%, respectively. This is equivalent to a MERV rating of 15 and 16, respectively (see Figures S3 and S4 in the Supporting Information). Test Vehicle Selections. Twelve passenger vehicles of different models and types from different manufacturers and countries of origin were selected to investigate the in-cabin exposure reductions resulting from the application of the HECA filters. As listed in Table 1, the vehicle selection included two hatchbacks, six sedans, two sport utility vehicles (SUVs), and two minivans. The vehicle models were recruited among popular vehicle models. The cabin volume size ranged 2329

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Figure 1. In-cabin reduction of (a) UFPs, (b) BC, and (c) PM2.5 with the application of different filters and under different driving conditions: stationary (white), local roadway (gray), and freeway (black). Each pair of symbols and error bars represent the mean and standard deviation of 1min averaged data from the 12 test vehicle models. There were at least 150 observations averaged from more than 9000 one-second raw data for each pair.

reduction of 93% and up to 99% for UFPs. The application of the HECA A filter offered an average reduction of 68−82%. In comparison, the no-filter scenario showed an average reduction of 20−55% and the in-use OEM filter had an average reduction of 35−70%. In addition to the great in-cabin UFP reduction, the application of the HECA filters minimized the variability (i.e., error bars) of UFP concentrations among different vehicle models. Figure 1 also illustrates in-cabin pollutant reductions under each of the driving conditions (i.e., stationary, local, and freeway). As shown in Figure 1a, the maximum UFP reduction occurred on freeways but the in-cabin reduction was smaller under stationary conditions. One should note that the greater reduction observed in the freeway environment was not because of lower in-cabin UFP concentration during the freeway testing. The overall in-cabin concentrations were still the highest under freeway conditions, followed by the local, and then stationary conditions. Instead, this greater reduction is likely because aerosols from different sources have different size distributions and the size-dependent filtration efficiency of the tested filters in different scenarios (see Particle Size Distributions and Size-Segregated In-Cabin UFP Reduction sections for details). Figure 1b,c presents another interesting observation. The incabin pollutant reductions were lower for BC and PM2.5 than UFPs, and lower on freeway than on local roadway. This is likely due to three factors. First, BC and PM2.5 reflect particle mass concentrations; therefore, the diffusion loss of smaller particles has little effect on BC and PM2.5 reduction. Second, the mass concentrations of BC and PM2.5 represent a wider range of particles with diameters up to 1 and 2.5 μm, respectively. Larger particles dominate BC and PM2.5 measurements and any unfiltered ones may lead to a decrease of total in-cabin reduction. Finally, the smaller in-cabin reduction under freeway conditions is in part due to the increase of the infiltrated (i.e., unfiltered) portion of the on-road pollutants, which often occurs at higher driving speeds on freeways. The infiltration effects were less noticeable for UFPs (Figure 1a) because the diffusion loss during the infiltration process was also significant for the nucleation mode UFPs on the freeways. However, the infiltrated on-road BC and PM2.5 can lead to a substantial increase in the overall in-cabin particle mass concentration. Because of these three factors, the in-cabin reductions of BC and PM2.5 were smaller than those of UFPs

distribution in the size range 7.37−289 nm were concurrently collected. The applied scanning and retrace times were 100 and 20 s, respectively. Each pair of instruments was collocated before and after the field sampling for data quality assurance and good correlation with little bias was observed (see Figure S5 in the Supporting Information). Under the medium fan setting in OA mode, data were collected for each test vehicle model under three different driving conditions: stationary and on local roadway and freeway. Stationary sampling was conducted in a parking lot. Local-roadway tests were conducted on the 3-mile sector of Westwood Boulevard between Wilshire Boulevard and National Boulevard in Los Angeles. The freeway testing route included a 22-mile segment of I-405 between the I-10 and I-710 freeways. The test segment of I-405 has heavy traffic of approximately 67 000 vehicles/day; however, hourly traffic volume flow rate was reasonably consistent (i.e., 3623 ± 99 vehicles/h) throughout all testing periods. Ambient temperature and relative humidity were 23 ± 4 °C and 50 ± 19%, respectively. In-cabin air temperature and relative humidity were 21 ± 3 °C and 70 ± 6%, respectively. Four different filtration scenarios (i.e., no filter, in-use OEM, HECA A, and HECA B) were examined under the three different driving conditions for 15−20 min each. The collected data covered 144 different experimental conditions and included more than 130 000 pairs of 1 s concentration data concurrently acquired for both in-cabin and on-road for each pollutant.

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RESULTS AND DISCUSSION In-Cabin Reduction of UFPs, BC, and PM2.5. Figure 1 provides average in-cabin pollutant reductions relative to the on-road ambient concentration for UFPs, BC, and PM2.5. The reduction was calculated from the concurrently measured incabin/on-road (I/O) concentration ratio (i.e., in-cabin reduction = 1 − I/O). The data points are the means of the 1-min averaged data for each of the 12 test vehicle models for (a) UFPs, (b) BC, and (c) PM2.5, under different driving conditions and different filtration scenarios. For each plotted data point (i.e., 1-min mean) and error bar (i.e., standard deviation), there were at least 150 observations averaged from more than 9000 one-second raw data. As shown in Figure 1a, under both stationary and realistic driving conditions, the HECA B filter achieved an average 2330

dx.doi.org/10.1021/es404952q | Environ. Sci. Technol. 2014, 48, 2328−2335

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Figure 2. Averaged particle size distribution data collected for the on-road and in-cabin environments using different filtration scenarios (i.e., no filter, in-use OEM, and HECA B) under (a) stationary, (b) local roadway, and (c) freeway conditions. The size distribution data for the HECA A case were not shown for clarity.

range for all three driving conditions. Even with no filters, the reduction was observable. Installation of in-use OEM filters offered additional particle removal for all conditions, but the reduction remained small in magnitude (Figure 2a−c). Upon retrofitting with HECA B filters, the in-cabin particle concentration was further decreased, especially for particles in the nucleation mode as in the case of the freeway driving shown in Figure 2c. As previously discussed in Figure 1a, the maximum reduction of the in-cabin UFP number concentration occurred on freeways. The particle size distribution data in Figure 2c offered explanation. The in-cabin reduction (i.e., 1 − I/O) is sensitive to the ambient particle size distribution, especially for UFP number concentrations. As seen in Figure 2c, the freeway aerosol was dominated by nucleation-mode particles with a mode diameter near 30 nm. These smaller UFPs contribute greatly to the total particle counts. Meanwhile, the filtration efficiency of the HECA B filter is also much higher in this size range compared with that of other filters (see Figure 3). This is likely due to its smaller fiber diameter, which enhances particle collection by interception.29 Consequently, the HECA B filter was more effective for the nucleation mode particles under freeway conditions. The following section discusses the sizespecific UFP removal efficiency in greater detail. Size-Segregated In-Cabin UFP Reduction. Figure 3 shows the average UFP removal efficiency as a function of

and smaller under freeway driving conditions than the local driving conditions. Nevertheless, the HECA filters successfully reduced all three pollutants under the experimental conditions. With respect to the on-road concentrations, the HECA B filter reduced the incabin concentrations of UFPs, BC, and PM2.5 by approximately 93%, 80%, and 70% on average, respectively. The use of the HECA B filter also greatly reduced the variability in the data from different vehicle models and under different driving conditions. Compared to the in-use OEM filters, the HECA filters increased the removal of the three pollutants by a factor of 2−3. Table S1 in the Supporting Information summarizes the measured UFPs, BC, and PM2.5 concentrations under all experimental conditions. Particle Size Distributions. Figure 2 shows an overview of the in-cabin and on-road particle size distributions averaged across each sampling period for each filter type under different driving conditions, namely, (a) stationary, (b) local, and (c) freeway. The solid lines represent the particle size distribution data collected for the on-road air, whereas the dotted-anddashed lines represent the in-cabin particle size distributions with no filter, the in-use OEM filter, and the HECA B filter. For clarity, Figure 2 excluded the size distribution results for the HECA A filter, which is similar to those of the HECA B filter but slightly higher. The three driving conditions provided distinctively different on-road particle size distributions. For stationary conditions, the ambient particle size distributions had a mode diameter of ∼80 nm, which was larger than ∼30 nm observed on the freeway. Because of the abundant presence of nucleation mode particles, the on-freeway conditions exhibited a typical bimodal size distribution (Figure 2c). In comparison, the data collected on local roadways showed a mixture of the stationary and freeway particle size distributions. The particle size distribution had three distinctive modes (Figure 2b) because measured particle size distribution experienced changes in the mode diameter and resulted in multiple mode diameters in the average size distribution. The multiple modes reflect the complexity of the changing traffic density and vehicle emissions due to the stop-and-go traffic pattern on local streets. The in-cabin reductions were commonly found across a wide range of particle sizes for all driving conditions. In comparison with the ambient concentrations, the data from different filtration scenarios demonstrate a substantial reduction of incabin particle concentrations across the measured particle size

Figure 3. Comparison among size-resolved particle removal efficiency for HECA B, HECA A, in-use OEM, and no-filter cases. The plotted data are the averages across all field conditions (i.e., stationary, local roadway, and freeway) for the specified filtration scenario. 2331

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Figure 4. Time- and size-resolved UFP concentration contour plots for (a) stationary ambient, (b) stationary in-cabin, (c) freeway ambient, and (d) freeway in-cabin conditions. The color intensity is the normalized particle concentration (dN/d Log Dp) using a log scale, with units of particles/ cm3. In-cabin data were collected with the HECA B filter installed.

particle size. The in-cabin UFP removal for each filter type is the average of the field measurements under all three driving conditions. Unlike laboratory filtration efficiency tests using KCl aerosols (see Figures S3 and S4 in the Supporting Information), field measurements used actual particles from different roadways. Thus, data presented in Figure 3 should be distinguished from the standardized laboratory testing that uses laboratory-generated particles under fixed flow rates. Across the measured size ranges (10−200 nm), the in-cabin UFP reduction was most effective when using the HECA B filter, followed by the HECA A filter, the in-use OEM filter, and no filter. Since particle diffusion loss occurs in the ventilation system, reductions were observed even with no filter installed. Figure 3 indicates the effectiveness of in-cabin particle reduction can be different across different sizes. For instance, the no-filter case exhibited a significant decrease of removal efficiency from 60% to 20% as the particle size decreased from 100 to 10 nm. Although the removal efficiency for the in-use OEM filter was relatively consistent at a level of 60−65% across the measured size range, it decreased considerably to 35% for particles smaller than 15 nm. Unlike no-filter and in-use OEM filter cases, the removal efficiency of the HECA B filter was consistently high at approximately 95% for particle sizes down to 50 nm, with only a slight decrease down to 85% for particles smaller than 50 nm. In comparison, the HECA A filter had a UFP removal of 75− 93%. Both HECA filters consequently offered more consistent

particle removal efficiency across the measured size range with much less variability than no filter and OEM filters. Therefore, under field conditions used in this study, the in-use OEM filter could not effectively remove particles smaller than 50 nm, whereas HECA B filter provided highly effective and consistent particle removal across the measured size range. Different size-specific removal efficiencies in each filtration scenario also explain why the in-cabin UFP removal was higher on freeways (Figure 1a). The minimum filtration efficiency usually occurs around 0.1−0.3 μm (i.e., the most penetrating particle size) for conventional fibrous filters (e.g., in-use OEM filters used in this study).29 Thus, using the in-use OEM filter, the overall in-cabin reduction is expected to decrease for the larger particles (i.e., mode diameter of ∼80 nm) observed under stationary conditions. Conversely, more reduction can occur for smaller particles (i.e., mode diameter of ∼30 nm) in the freeway environment even with the in-use OEM filter. In the no-filter scenario, the filtration theory for fibrous filters does not apply. Since mechanical ventilation system delivered onroad ambient particles at higher speed in the absence of filter resistance, those particles including nucleation mode particles had less time to diffuse to the surface of the ventilation system. Consequently, the application of HECA B filters offered consistent particle removal efficiencies across the measured particle size range and achieved an in-cabin UFP reduction by 93% on average in the field. Its performance is less affected by the on-road particle size distribution and is 2−3 times better 2332

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HECA filters. The changes in the ventilation airflow rates were estimated with respect to those measured with the in-use OEM filters under stationary conditions. With the in-use OEM filters, the airflow rate was estimated to be 306 m3/h on average (±101 m3/h) across the test vehicle models under stationary conditions. The presented data are the means and standard deviations of all tested vehicle models under different field conditions. A nonparametric Mann−Whitney U-test was used to examine the differences of ventilation airflow rate under various filtration scenarios. As expected, retrofitting with HECA B filters significantly reduced the ventilation airflow rate for all driving conditions (p < 0.01). In comparison with the in-use OEM, the ventilation airflow rate was reduced by 22%, 12%, and 7% on average under the stationary, local, and freeway conditions, respectively. However, these reductions are unlikely to cause thermal comfort issues to passengers under realistic driving conditions. In addition, the passive ventilation significantly increases the ventilation airflow rate when driving on freeways,30 which helps to overcome the additional pressure drop from installing HECA filters. Simultaneous Mitigation of UFPs and CO2. Retrofitting the HECA filters enable the simultaneous mitigation of in-cabin UFPs and CO2. Figure 6a,b shows under stationary conditions, the time series of I/O estimates for UFPs with in-use OEM and HECA B filters, as well as CO2 in OA and RC mode, respectively. The plotted data are means and the shades are the standard deviations for all test vehicles. As shown in Figure 6b, operating vehicles in the RC mode reduced the UFP I/O down to 0.07 even with in-use OEM filters. However, this approach also imposes the unwanted problem of passenger-exhaled CO2 accumulation. Within 15 min, the CO2 I/O tripled with 1−2 passengers (i.e., 1.3 passengers on average) inside the stationary vehicle cabin. The tripled CO2 I/O is equivalent to a concentration of 2500−4000 ppm on average. Although the application of a HECA filter in RC mode reduced the in-cabin UFPs further than that of the in-use OEM filters, the problem of CO2 accumulation still remained at a similar magnitude (data not provided for clarity). To solve this problem, this study utilized the OA-mode ventilation system to supply HECA-filtered ambient air into the passenger cabin (Figure 6a). Under stationary OA-mode conditions with 1.3 passengers on average, the in-cabin CO2 concentrations were maintained at a reasonable level that was ∼20% higher than the ambient. With two passengers driving on local streets and freeways, the average in-cabin CO 2 concentrations were 69% (i.e., 790 ppm) and 58% (i.e., 750 ppm) higher than the ambient concentration (i.e., ∼470 ppm). Table S1 in the Supporting Information tabulates the CO2 concentration data in more detail. In the meantime, the in-cabin UFP concentrations also decreased and UFP I/O stabilized at 0.07 in the OA mode with the HECA B filter. Compared with the in-use OEM filter, which had a UFP I/O of 0.60, the application of the HECA B filter achieved a substantially lower (i.e., 0.07) in-cabin UFP I/ O ratio. Regardless of the ambient concentration fluctuations under local and freeway driving conditions, the in-cabin UFP concentration in the HECA B filter scenario was an order of magnitude lower than the on-roadway level. In summary, by retrofitting the existing automotive ventilation systems with the developed HECA filters, this study demonstrated an average in-cabin UFP reduction of 93% and up to 99% under the field conditions. The level of pollutant

than that of the in-use OEM filters. These findings also suggest that a large portion of the measured in-cabin UFPs are in the smaller size range, even after the filtration process with in-use OEM filters. Since the deleterious health effects of UFPs are related to smaller particles, a consistent removal across a wide range of particle sizes is important and can be readily achievable via in-cabin HECA filters. Instantaneous UFP Reduction. As shown in Figure 4, the reduction of in-cabin UFPs occurs almost instantaneously when HECA B filter was used. Time-resolved UFP size distributions measured inside (Figure 4b,d) and outside (Figure 4a,c) of vehicles are shown as contour plots in Figure 4, where the xaxis presents the elapsed time at which data were collected, the y-axis is the particle size in log scale, and the color intensity indicates normalized particle number concentration (dN/d Log Dp) for a given size at a given time. The same concentration scale was used for all plots. Under both stationary (Figure 4b) and freeway-driving (Figure 4d) conditions, the use of HECA B filters offered a strong in-cabin UFP reduction throughout the measured size range. The reduction occurred immediately after the HECA B filter was installed and continued throughout the measurement period. With HECA B filter installed, the particle size-specific reduction was approximately 1 order of magnitude higher across the measured size range and was especially effective for nucleation mode particles. This reduction is similar to the 93% reduction of in-cabin UFP number concentrations (Figure 1) and occurred for all driving conditions including local roadways (data not shown here). Ventilation Airflow Rate Reduction. The installation of HECA filters may result in a large pressure drop that reduces the ventilation airflow rate into the passenger cabin. Because the automotive ventilation systems primarily serve to offer thermal comfort to passengers, the reduction in the airflow rate could become a critical limitation for in-cabin HECA filter application. Figure 5 summarizes changes of ventilation airflow rates and in-cabin UFP reductions upon retrofitting with the in-cabin

Figure 5. Changes of the ventilation flow rates vs in-cabin UFP reduction under different driving conditions for different filtration scenarios: HECA B, HECA A, in-use OEM, and no filter. The changes were estimated relative to the ventilation flow rate measurements with the in-use OEM filters under stationary conditions for each vehicle as indicated by the arrow (i.e., 306 m3/h on average, with a standard deviation of ±101 m3/h). The symbols and error bars are the averages and standard deviations of the relative changes, respectively. 2333

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Figure 6. I/O ratios as a function of time for UFPs (dots) and CO2 (line) using the HECA B and in-use OEM filters under (a) OA and (b) RC mode under stationary conditions with 1.3 passengers inside vehicle cabins on average. The symbols represent the averages of 1 min data from all test vehicle models. The shading represents the standard deviations.

authors and do not necessarily reflect the views of the California ARB or the National Science Foundation.

mitigation was slightly lower for BC and PM2.5. The percentages of in-cabin pollutant reduction were higher on the freeways than on local roadways and under stationary conditions. Overall, the developed HECA filters achieved 2−3 times greater reduction than the in-use OEM filters. Using the HECA filters in the OA-mode ventilation also maintained the in-cabin CO2 concentration at 630−920 ppm with two passengers driving on local streets and freeways. These concentrations are much less than those in the RC mode, which could reach 2500−4000 ppm. In conclusion, the OA mode ventilation system retrofitted with in-cabin HECA filters can be highly effective for the simultaneous reduction of incabin passenger exposures to UFPs and CO2.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2 display SEM images of HECA A and HECA B filters. Figures S3 and S4 present lab testing results for the two filters. Figure S5 shows instrument collocation data. S6 (Table S1) tabulates the measurements in this study. Figure S7 provides the changes in CO2 I/O ratios in no-filter and HECA A filter scenarios. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 310-825-4324; fax: 310-794-2106; e-mail: Yifang@ ucla.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study complements the work in progress that is partially supported by the California Air Resources Board (ARB) under contract #11-310 and the National Science Foundation’s CAREER Award under contract #32525-A6010 AI. The authors thank Frank Hammes and Mark Mihara at the IQAir for their collaboration in developing the in-cabin HECA filters and providing technical data. The authors also thank David C. Fung, Claire Y. Kim, Nu Yu, and David C. Quiros for their assistance in the field sampling. Mention of trade names or products does not constitute an endorsement or recommendation for commercial use. Any opinions, findings, conclusions, or recommendations expressed in this report are those of the 2334

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